Abstract: Conversion electrodes typically have high theoretical specific capacity, but mostly suffer large structural changes during charge/discharge and result in poor cycling stability. The optimization of the polycrystalline materials is the mostly used strategy, however, these polycrystalline materials are intrinsically vulnerable to grain-boundary (intergranular) fracture caused by the anisotropic volume change during sodiation/desodiation, resulting in rapid impedance growth and capacity decay. Herein, we propose an alternative pathway to design single-crystal materials as potential conversion anodes. As an example, SnO₂ with different crystallinities is successfully synthesized via solvothermal methods and compared to determine the implications of different crystallinity for the electrochemical properties of conversion anodes. It is demonstrated that the single-crystal SnO₂ not only has faster Na⁺ diffusion dynamics but also maintains structural stability via topotactic reaction. Further optimization of the electron conduction and structural robustness is realized by uniformly covering a graphitic carbon shell on the surface of single-crystal SnO₂ nanosheets. The modified single-crystal SnO₂ exhibits a high reversible capacity of 436.2 mA h g^-1 and maintains a high capacity of 257.1 mA h g^-1 and remarkable capacity retention of about 98.9 % after 9000 cycles at 5000 mA g^-1. The deep understandings of the topotactic reaction in single crystal conversion anode in this work provide a theoretical foundation and new direction for further developing electrode materials with excellent electrochemical performance, especially high rate capabilities, and long cyclability.

Key words: Conversion electrodes, Single-crystal, SnO2, Topotactic reaction, Na-ion batteries